Historical forms of navigation include pilotage, dead reckoning, celestial navigation, radio navigation, and inertial navigation. Radio navigation relies on radio-frequency sources with known locations, such as Global Positioning System (GPS) satellites or terrestrial airport beacons, to determine where one is relative to where one wants to go. Inertial navigation systems rely on knowing one's initial position, velocity, and attitude and measuring attitude rates and accelerations to provide an estimate of one's position after movement away from the initial position. Integrated systems further improve performance by combining the high-bandwidth motion control of inertial navigation with the long-term position location accuracy of GPS. See M. S. Grewal et al., Global Positioning Systems, Inertial Navigation, and Integration, John Wiley & Sons, New York (2001).
GPS is a satellite-based navigation system that provides continuous positioning and timing information anywhere in the world under all weather conditions. GPS nominally uses at least 24 operational satellites with known relative locations, each satellite in one of six nongeostationary, circular orbits about 2200 km high above the earth. These orbiting satellites continuously broadcast high frequency radio signals to passive receivers on earth. Pseudoranges, or estimated distances from the receiver to the satellites, are computed from the relative time delay of the received signals from several of the satellites. Normally, information is required from four satellites to synchronize the receiver's internal clock with the highly accurate GPS satellite clocks and enable a precise position fix to be determined from the pseudoranges by trilateration. Positioning accuracy for civilian applications is nominally tens of meters. Differential techniques can significantly improve the accuracy of GPS-derived positions by using tandem receivers to simultaneously track the same GPS satellites, thereby enabling submeter-positioning accuracy. GPS has many attractive features, such as inexpensive and light-weight receivers, high positional accuracy in three dimensions, global coverage, all-weather capability, and availability to an unlimited and diverse set of users. However, GPS obviously requires access to the satellite radio signals and such signals may not be available in some locations or may be vulnerable to electronic jamming.
Alternatively, inertial navigation can be used for active position measurement. Inertial navigation systems typically use an inertial measurement unit (IMU) comprising a cluster of sensors to provide full three-axis attitude measurements. For example, the cluster can comprise three accelerometers for measuring acceleration in three directions and three gyroscopes for measuring rotation rate and to provide a reference frame for the accelerometers. The acceleration can be doubly integrated and the rotation rate can be singly integrated by a navigation computer to provide position. The IMU can be a gimbaled system, wherein the sensor cluster is mounted in a gimbal frame to isolate the sensors from external rotations of the host vehicle and keep them level and pointing in a fixed direction via torquing devices. Alternatively, the IMU can be a strapdown system, wherein the sensor cluster is strapped down to the frame of the host vehicle and the measurements are made in reference to the vehicle axes, for example, the roll, pitch, and yaw axes of an airplane in flight.
Inertial navigation systems have some advantages over other forms of navigation, such as GPS. Since IMUs are autonomous, they do not rely on external navigation aids, such as landmarks, celestial objects, or satellites, and they can be used underwater or in underground tunnels. Furthermore, since IMUs are completely on-board the host vehicle, the accelerations and rotation rates can be easily integrated with guidance and control systems and they are inherently stealthy and immune to jamming. However, acquisition, operating, and maintenance costs, system size and weight, and power requirements tend to be higher with IMU systems than with GPS systems. Most importantly, measurement drift, due to the integration of IMU acceleration and rotation rate errors, can cause navigation errors to grow dramatically with time.
There are currently many methods to tracking objects that move relatively fast, such as fixed wing aircraft. However, tracking the movement of objects that move intermittently remains a problem in situations where GPS is passively or actively denied and access to reference beacons is not otherwise available. In particular, IMUs are most useful in situations in which the time duration of the object's motion is sufficiently short such that the accumulated drift error is negligible. IMUs may also be useful in integrated navigation systems in which access to sky or terrestrial reference beacons is available periodically to provide aiding for the IMU, allowing for reinitialization of the IMU position to compensate for the measurement drift. However, without access to these references, even perfect initial location information coupled with a perfect IMU would quickly become useless for locating slow-moving or objects at rest. In particular, there are no systems of any kind that can continuously and reliably monitor the latitude and longitude of an intermittently stationary object without access to external reference beacons or satellites.
The geophysics-based method of present invention overcomes the limitations of conventional navigation systems by analyzing the fundamental properties of earth gravity and spin to obtain a fix on the position of a stationary earth object. The measurement of geophysical phenomenon enables the determination of latitude and longitude without the need for external infrastructure (e.g., satellites needed for GPS) or foreknowledge of how the object moves around or the general area it is in (e.g., as required for inertial navigation systems). Because gravity cannot be shielded and acts at long distances, the geophysics-based method can determine object position in underground and shielded locations. It does not require visual or infrared access to the sky, as in celestial navigation or GPS. It also is immune to RF jamming or magnetic jamming. Nor does the present invention exhibit the time-dependent drift error problems of IMUs.